A vehicle power transmission is an important part of a vehicle power train. The primary function of a vehicle power transmission is to regulate vehicle speed and torque delivered to the driven wheels from a driving engine to meet operator demands for speed and acceleration. The major requirements for vehicle power transmissions are speed ratio ranges, torque capacity, transmission and system efficiencies, weight, and cost.
There are two types of conventional vehicle power transmissions: stepwise and step-less. Stepwise transmissions, using multiple gear sets and clutching devices, are quite popular. The speed ratio changes are accomplished in discrete steps by engaging different gears in the power transmission pathway. Speed ratio changes are often associated with interruptions in both speed and torque. The output speed variation between two speed ratios is realized by varying the input speed supplied by the driving engine. A major disadvantage of a stepwise vehicle power transmission is system efficiency, since the engine cannot always operate at its most efficiency speed. For the same reason, pollution is also a problem for a vehicle with a stepwise power transmission.
Step-less transmissions provide a continuously variable speed ratio change. With a step-less transmission, it is possible to operate a driving engine at an optimal speed and, therefore, keep the engine at its peak efficiency. Common types of step-less transmissions include hydrostatic drives and friction drives or traction drives (i.e. toroidal drives, belt drive continuously variable transmissions (CVTs)).
Hydrostatic traction drives have several drawbacks. The hydrostatic traction drives are noisy and have low efficiency, and as such, they generally are used only for low speed applications such as agriculture machines and construction equipment. Traction drives are more efficient, but they are less rugged for handling large torque loads. Overall, many traction drives are usually quite heavy and costly to manufacture.
Recent developments in step-less transmissions has been in the area of electro-mechanical transmissions, such as European Granted Patent No. EP 0755818 B1 and Tenberge, P., (1999), “Electric-Mechanical Hybrid Transmission,” Proc. International Congress on Continuously Variable Power Transmission, Eindhoven University of Technology (hereinafter “Tenberge”).
Most of the newly proposed electro-mechanical transmissions operate on a power-split concept historically developed for hydrostatic drives. In a power-split transmission, there exists multiple parallel power paths. There are two basic power-splitting devices, a single planetary unit and a compounded planetary unit that comprises two nested sub-planetary sets. When properly connected with two electric machines, a single planetary electro-mechanical transmission is capable of producing at least one point in speed ratio where no power is passing through the electric machines and all power transmitted is passing through a mechanical path. This point is referred to as the mechanical node point. For an electro-mechanical transimssion there is no energy conversion at the mechanical node point from mechanical form to electric form and back to mechanical form. Thus, the transmission yields the maximum efficiency. An electro-mechanical transmission with a single planetary train is called single node system. An example of such a system is the Toyota Hybrid System now in limited production.
However, as the output-to-input speed ratio of the transmission moves away from the node point, the power to the electric machines in a single-node system increases significantly. The power that is circulated between the two electric machines can far exceed the power that the transmission is transmitting. Such internal power circulation occurs at speed ratios either above the node point when one motor is connected to the output shaft or below the node point when one motor is connected to the input shaft. Internal power circulation generates heat and power loss and offsets the efficiency benefit otherwise provided by the transmission. For this reason, the effective speed ratio range is limited. To cover a useful speed ratio range, oversized electric machines are often used.
To reduce or restrict internal power circulation, sophisticated control systems were developed for the Toyota Hybrid System. These control systems monitor the torque value of the electric motor and shift the driving engine to another driving point of higher speed. In other words, the control system limits the output-to-input speed ratio to the node point or slightly above.
In contrast to a single-node system, an electro-mechanical transmission with a compound planetary unit is considered a two node system which contains four branches. When two of its four branches are connected to two electric machines, it can produce at least two mechanical node points where no electric power is passing from the input of the transmission to the output through the electric machines. As with single planetary unit, a two-node system also suffers from the internal power circulation problem. Internal power circulation occurs outside the two node points, below the first node point or above the second node point. But in general, a two-node system has a wider speed ratio range than a single node system.
To extend the speed ratio range and overcome excessive internal power circulation, multi-regime (also called multi-mode) infinitely variable transmissions, analogous to speed ratio shifting in stepwise transmissions, have been proposed.
Various configurations of variable, two-mode, power split, parallel, hybrid electric transmissions are also known. They all employ at least a compound planetary set along with other gears and shifting devices and two electric machines. The two-mode design provides adequate speed ratio range where the first mode covers slow vehicle speed operation and the second mode covers relatively high-speed operation. The mode shifting in a two-mode design is achieved through the use of clutches and synchronized gear sets, resulting in a complex design.
In the first mode, there exists a pure mechanical node point. In the second mode, there are two mechanical node points. At each mechanical node point, there is no energy conversion from mechanical form to electric form and back to mechanical form. Thus, the transmission operates at maximum efficiency.
Away from the node points, the power to the electric machines increases. In fact, the power to electric machines increases rapidly as the vehicle's speed drops below the first node point in the second mode operation. Therefore, the transmission has to go through a mode shifting in order to configure for slow speed operation. As mentioned before, this shifting requires synchronizing gear sets. Although the shifting is continuous in speed, it is not continuous in torque and power.
Shifting between different modes presents an interesting challenge. It is often associated with a torque and a power interruption. Various means have been disclosed in prior art to perfect synchronizing mechanisms. To reduce torque interruption due to torque reversals in electric machines, Tenberge presented a means of using electronically controlled hydraulic clutch and brake packs to retain the torque balance and facilitate the mode shifting through differential engagement.
U.S. Pat. No. 6,203,468 illustrates a speed and torque control method to prevent speed and torque fluctuations during mode switching from series drive to parallel drive. The basic strategy is to match the speeds of the two electric machines and reduce the driving engine torque to zero at the switching point. Since the driving engine operating at switching point produces zero power, an on-board energy storage device is required for such system.